In general in heavy-goods vehicle tires, the carcass reinforcement is anchored on either side in the region of the bead and is surmounted radially by a crown reinforcement consisting of at least two superposed layers formed from threads or cords that are parallel in each layer and crossed from one layer to the next, making angles of between 10° and 45° with the circumferential direction. Said working layers, forming the working reinforcement, may further be covered with at least one protective layer formed from advantageously extensible metal reinforcing elements, called elastic elements. It may also comprise a layer of low-extensibility metal threads or cords making an angle of between 45° and 90° with the circumferential direction, this ply, called triangulation ply, being located radially between the carcass reinforcement and the first crown ply called the working ply, these being formed from parallel threads or cords at angles of at most equal to 45° in absolute value. The triangulation ply forms, with at least said working ply, a triangulated reinforcement which undergoes, when subjected to the various stresses, little deformation, the essential role of the triangulation ply being to take up the transverse compressive forces to which all of the reinforcing elements in the crown region of the tire are subjected.
In the case of heavy-goods vehicle tires, a single protective layer is usually present and its protecting elements are, in most cases, oriented in the same direction and at the same angle in absolute value as those of the reinforcing elements of the radially outermost, and therefore radially adjacent, working layer. In the case of civil engineering vehicle tires, intended for running on more or less uneven ground, the presence of two protective layers is advantageous, the reinforcing elements being crossed from one layer to the next and the reinforcing elements of the radially inner protective layer being crossed with the inextensible reinforcing elements of the radially outer working layer adjacent to said radially inner protective layer.
The circumferential direction, or longitudinal direction, of the tire is the direction corresponding to the periphery of the tire and defined by the running direction of the tire.
The transverse or axial direction of the tire is parallel to the rotation axis of the tire.
The radial direction is a direction cutting the rotation axis of the tire and perpendicular thereto.
The rotation axis of the tire is the axis about which it rotates in normal use.
A radial or meridian plane is a plane that contains the rotation axis of the tire.
The circumferential median, or equatorial, plane is a plane perpendicular to the rotation axis of the tire and that divides the tire into two halves.
Certain current “road” tires are intended to run at high speed on increasingly long journeys, because of the improvements in road networks and the growth of motorway networks throughout the world. All the conditions, under which such a tire is called upon to run, without doubt enable the tire to be run for a larger number of kilometers, since the wear of the tire is less. However, the endurance of this tire is prejudiced. To permit one or even two retreading operations on such tires, so as to extend their lifetime, it is necessary to preserve a structure and especially a carcass reinforcement with endurance properties which are sufficient to withstand said retreading operations.
Prolonged running under particularly severe conditions of tires thus constructed effectively introduces limits in terms of endurance of these tires.
The elements of the carcass reinforcement are in particular subjected to flexural and compressive stresses during running which adversely affect their endurance. The cords that make up the reinforcing elements of the carcass layers are in fact subjected to large stresses when the tires are running, especially to repeated flexural stresses or variations in curvature, leading to friction between the threads, and therefore wear and fatigue: this phenomenon is termed “fatigue fretting”.
To fulfill their function of strengthening the carcass reinforcement of the tire, said cords must firstly have good flexibility and a high endurance in flexure, which means in particular that their threads have to have a relatively small diameter, preferably less than 0.28 mm, more preferably less than 0.25 mm, generally smaller than that of the threads used in conventional cords for the crown reinforcements of tires.
The cords of the carcass reinforcement are also subject to the phenomenon of “fatigue-corrosion” due to the very nature of the cords, which promote the passage of corrosive agents such as oxygen and moisture or even drain said agents. Specifically, air or water penetrating the tire, for example as a result of degradation following a cut or more simply because of the permeability, albeit low, of the inner surface of the tire, may be conveyed by the channels formed within the cords because of their very structure.
All these fatigue phenomena, which are generally grouped together under the generic term “fatigue-fretting-corrosion”, are the cause of progressive degradation of the mechanical properties of the cords and may, under the severest running conditions, affect the lifetime of said cords.
To improve the endurance of these cords of the carcass reinforcement, it is known in particular to increase the thickness of the rubber layer that forms the internal wall of the cavity of the tire in order to minimize the permeability of said layer. This layer is usually composed partly of butyl rubber so as to better seal the tire. This type of material has the drawback of increasing the cost of the tire.
It is also known to modify the construction of said cords so as in particular to increase their penetrability by the rubber and thus limit or even eliminate the passage of oxidizing agents via the channels formed within the cords. Tires produced in this way have revealed problems of air pockets appearing during manufacture of the tire.
This is because the various manufacturing steps lead to the formation of occluded air pockets. In the case of tires comprising a carcass reinforcement formed from cords having a structure that forms channels able to conduct the air, these air pockets disappear owing to the diffusion of the air into the materials, especially through said channels existing within the cords. In the case of tires comprising a carcass reinforcement formed from cords having a structure which is highly penetrated by the rubber, these air pockets remain after the manufacturing steps. What appears is only a displacement of these air pockets during the step of curing the tire, said pockets being displaced towards regions where a low pressure is exerted. The displacement of the air takes place along the carcass reinforcement, along passages that exist between the reinforcing elements, the layers of rubber compound covering the reinforcing elements forming indentation zones parallel to the reinforcing elements before the step of curing the tire. These indentation zones thus permit the air to move slightly depending on the pressure that is exerted on the regions where the air pockets exist. The pressure or the pressure variations occur especially during the step of curing the tire or else during the conforming step, if it exists.
The appearance of these air pockets is usually unacceptable depending on their location, and may require scrapping the tires, as said air pockets may become zones of weakness in the tire. The manufacturing costs therefore become unacceptable simply because of the poor production yields.